This invention relates to the centering of an article about its true rotational center, and, more particularly, to a mechanical apparatus and method to achieve such centering.
In a number of fields, it is important to position an article so that it rotates precisely about its rotational center, taken to be its center of symmetry. In one example, optical lenses are sometimes ground to specifications of the wedge angle between the opposite faces of the lens. In order to measure the wedge angle, a lens must be positioned precisely so that its true center is known, and the lens is rotated about that center. For such applications, the center of rotation must be determined to an accuracy of better than about 1 micrometer.
The rotational centering of the lens is complicated by the fact that the lens is often not perfectly round in transverse section perpendicular to the axis of rotation, due to limitations in the manufacturing processes. That is, the lens may be fabricated such that it is slightly oblong in transverse section. The lens therefore cannot be centered simply based on the measuring and positioning of its outer periphery.
The same problem of precise rotational centering arises in other fields as well. Rotating elements such as shafts, machine parts, flywheels, and other articles must be centered to avoid excessive vibration and damage during operation.
There is therefore a need for a highly accurate approach to the rotational centering of articles. The present invention fulfills this need, and further provides related advantages.
The present invention provides a method for the rotational centering of an article, and apparatus utilized in practicing the method. The present approach is highly precise, achieving centering to accuracies better than 1 micrometer. The apparatus tolerates misalignments that otherwise would prevent precise centering. The technique may be automated and used in an iterative fashion. The centering apparatus may be scaled to any required size, and the approach is therefore applicable to a wide range of centering problems.
In accordance with the invention, a centering apparatus used in conjunction with a workpiece having a periphery comprises a workpiece spindle operable to rotate the workpiece about a rotational axis, and a centering head. The centering head comprises a mount movable in a movement direction lying in a plane perpendicular to the rotational axis. The movement direction is generally a radial line extending outwardly from the rotational axis. The centering head further includes a swivel head pivotably supported on the mount and pivotable about a pivot axis parallel to the rotational axis. The swivel head has a pair of bearings spaced apart along a direction lying generally perpendicular to the movement direction, with each bearing having a bearing axis parallel to the rotational axis. Preferably, the centering head further includes a distance measuring device operable to measure the distance between the periphery of the workpiece and a measurement location on the mount.
A method for centering the workpiece comprises mounting the workpiece on the spindle, positioning the centering head at a location adjacent to the periphery of the workpiece, rotating the spindle to rotate the workpiece, and measuring a runout curve using the distance measuring device of the centering head. The runout curve has a constant component and a variable component. The method further includes moving the centering head along the movement direction until the bearings contact the workpiece, and pushing the workpiece with the bearings in a direction and by an amount so as to reduce the constant component of the runout curve toward zero. The amount and direction of pushing the workpiece may be precisely calculated and applied, or approximated and the method applied iteratively to reach the desired degree of accuracy of the rotational centering.
The centering head is structured so that it tolerates slight misalignments. It is otherwise difficult to align and move the centering head perfectly along a radial movement direction, to the accuracies required to achieve the desired centering. In the present approach, the pair of bearings on the centering unit provide a pushing engagement that remains operable even if the centering head is slightly displaced from or angularly misaligned with the exact radial direction from the rotational axis of the spindle.
Other features and advantages of the present invention will be apparent from the following more detailed description of the preferred embodiment, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of the invention. The scope of the invention is not, however, limited to this preferred embodiment.